From The Perry Mason TV Show Book
Lisa Bannister is ready to kill herself after Stefan Riker (played by Werner Klemperer—Colonel Klink of Hogan’s Heroes) arrives in the United States from East Germany. He knows that Lisa is the daughter of the head of the Communist party in that country and has been in the United States illegally. Lisa’s stepdaughter Doris vows to help Lisa and takes up a romantic relationship with Riker. Her plan works so well that the police use it as a motive when Riker is murdered.

Perry has to use kid gloves on this case. He not only has to clear Doris, but must keep Lisa's dirty laundry out of the courtroom.

We learn in this episode that Perry’s office address is Suite 904 in the Brent Building.
+ It was handwritten by the client & found in her wrecked car by Perry/Paul. Mike Bedard 4.29.15
++ Begging your pardon, but Doris Bannister brings the scrap of paper with Perry's office address scribbled on it with her when she visits Perry initially. The paper from which that scrap was torn is the paper found in the wrecked car. jfh 28May2018.